Conventional beverage makers such as coffee brewing machines have water storage tanks, commonly made of stainless steel, to hold water and heating rods with which to heat the water in the water storage tanks. The heating rods include tubes packed with sand and heat generating filaments. Heat generated by the filament is transferred to the sand and, then, to the water in the water tank, thereby heating the water.
Other conventional beverage makers include water boilers similar to the hot water storage tanks except that these boilers are held under pressure enabling the water to be heated to a higher temperature.
Theses conventional beverage makers, however, suffer from a number of drawbacks. For instance, they require a lengthy cold start period during which a cold water tank, or a boiler, filled with unheated water is heated. They also require a long recovery time when heated water is dispensed and, then, replenished with unheated water. In addition, the water quality tends to degrade over time when kept at a high temperature for prolonged periods of time.
In an effort to alleviate the above drawbacks, some of the conventional coffee brewing machines include on-demand water heating devices. These conventional on-demand water heating devices heat water only when requested. Conventional on-demand heating devices that produce small quantities of heated water include indirect electrical resistance heaters which are bonded to a water pipe. On the other hand, conventional on-demand heating devices that produce larger quantities of heated water include heating blocks which contain a coiled water tube and a coiled heating rod encased in a block of metal. The heating block is a thermal energy storage device to heat water on-demand as unheated water passes through the heating block. This requires a constant supply of electrical power to the heating block in order to maintain it at a certain temperature, thereby wasting electrical energy and losing thermal energy to its environment. In general, the conventional on-demand water heaters are inefficient, among other reasons, because they utilize the indirect resistance heating method.
The conventional heating devices discussed above are prone to fail prematurely due to calcification. According to a lab test performed on the conventional on-demand heating devices, the devices failed after about 5500 cycles due to excessive calcification. In addition, the conventional heating devices, due to the drawbacks outlined above, cannot produce heated water at a stable temperature which is a desirable feature in brewing some high quality beverages.
Instead of the conventional water heating method described above, direct electrical resistance (DER) heating methods have been developed for industrial uses. The DER method is also known as electroheating, in-line heating or ohmic heating. A conventional DER device includes a pair of electrodes and an electric power supplier for applying a high power, high frequency electricity to the electrodes. As an electrically conductive medium, such as meat or other food products, passes between the electrodes, electric currents flow through the medium which generate heat therein. The medium generates heat since it acts as a resistor.
Several references disclose the DER methods for heating different types of electrically conductive medium. For instance, U.K. Patent Application No. GB-A-2304263 (the "'263 application") discloses an electroheating, processing, pasteurizing and cooking liquid egg. In this electroheating method, liquid egg is pasteurized when it passes through a pair of electrodes while electric power is applied between the electrodes. This method, however, heats only one type of electrically conductive medium, the liquid egg, in a controlled production line. In addition, this method, as with other conventional DER heating methods, requires a high power, high frequency electrical power supplier which tends to be relatively expensive for non-industrial use.